Wooden Thread Profiles

[Ray Gardiner]

After reading all the fuss about Moxon/Schwarz twin screw presses/vises, I have succumbed to fashion and decided to make one..

The screws will be 2" diameter 3 tpi, still deciding on what wood to use, I have some english oak, that might be suitable, but walnut with it's crosslinked grain might be less liable to crack across the threads...

I'm making the tap first, it's just mild steel but I'm thinking of making provision for an O1 cutting insert.

Now the question. What is the best profile for wooden threads, I see the commercial taps are all 60 degrees, whereas I understand the traditional profile is a 90 degree form, what about square or "acme like" trapezoidal?

I imagine that a square thread profile would be weaker at the root of the tooth than either 60 or 90 degrees, since the thickness of the root of the square profile will be half the root thickness of the triangular profile, also the full force will be applied directly across the grain, whereas with a triangular profile the force across the grain will only be a portion of the total (depending on the angle) and there will be a component of the force also compressing the thread.

The shaker style vises with wooden threads used 90 degrees I believe, but I just watched a Roy Underhill video where he makes a thread box using a 60 degree V cutter?

The commerically available Beal threading kits use a 60 degree cutter?

Did they all get it wrong or is 60 degrees just as good as 90? Maybe I'm just easily confused.. (don't answer that)

Regards
Ray

[george wilson]

Threads should be 90º for the best results on wood. Other threading devices that are 60º are not correct for wood threads,and are possibly the result of not doing proper research,or it might be that it is difficult to get commercial machine shops who are equipped to cut 60º metal threads to comply with making 90º taps for wood. Then,they end up making 60º threads for the male thread also. It actually would be good if the tap made the threads in the hole 60º,and the male thread was 90º. Then,the tips of the male thread would not shear off,because all the pressure would be on the roots of the male thread.

Square threads will break off,so will Acme. They are weaker,especially the square threads,because they get pressure out on the tip ends,and break. I had to make a new,proper 6" diameter wooden thread for the bookbinder's standing press. Years ago,someone made them a press with square threads. The threads sheared off so much that the press became useless. My proper threads will last hundreds of years,literally,on a screw that large.

[Mark Baldwin III]

George...unless I'm reading you wrong, I believe you've got a contradiction going on. You said that 90 degree threads are best, then you state that square threads break easier? Could you clarify? (not trying to bust your balls, just making sure I'm not missing something)
I've never threaded wood. In metal, Acme threads (IMHO) are superior to 60 degree threads for anything taking any amount of force repeatedly. For example, Snap-on pullers have standard threads on them. After wearing them out, I got with my R&D guys, and we made improved pullers with Acme threads. The new pullers are incredible workhorses and they don't bind or gall like the standard 60 degree threads. The force vector acting on an angled thread (Acme's trapezoid, or a 60 degree) moves some of the force down to the root at the angle of the thread. At 90 degrees, the force goes straight back against the thread. Been a long time since my thread making class days, but that's how I remember it.

[george wilson]

You aren't busting my balls. You just don't get what I said. The wood threads are V threads,like ordinary metal threads. they just have a 90º angle on their sides,rather than the 60º angle that standard metal threads have. It makes them beefier than metal threads,not as tall,and proportionately thicker at their bases.

Acme threads and especially square threads in metal are good POWER TRANSMISSION threads,because they don't try so hard to force the nut open. On lathes,with split nuts that open and close to engage the lead screw,this is important,as you don't want a lot of tendency to force the half nuts apart. Gear pullers are a good application for acme threads. In the old days,they used to use square threads,but acme types are easier to cut,and easier to close split or half nuts on.

[Ray Gardiner]

Hi Mark,

No contradiction, George is referring to the angle as shown below, please excuse the drawing quality.
http://www.backsaw.net/pics/threadprofiles.jpg

The acme and square threads will be weaker across the root, section "a" in the drawing.

George,
I like the idea of slightly different angles on the tap versus the thread box, I will work along those lines
a bit and see how it works out.

Regards
Ray

[Paul Incognito]

Thanks Ray,
I was a little lost on that one, too.
Paul

[Mark Baldwin III]

George-I must have read your post at least 5 times thinking, "what the #%%# is this guy on about?" As soon as Ray's picture started scrolling up from the bottom of my browser, I felt a little stupid. That was my "DUH!" moment for the day. I knew I had my thread mechanics right, but my angles were wrong. Maybe if I turned my head 45 degrees to the screen while I read it;)
You should have seen me when I first tried to learn about ball screws, they had me scratching my head a little.

[jamie shard]

After reading all the fuss about Moxon/Schwarz twin screw presses/vises, I have succumbed to fashion and decided to make one..


This is great, hope you will post your progress!

[Jim Koepke]

Glad this came up. I have been thinking about cutting wood threads.

I also saw the Undehill video about making a thread cutter and it didn't seem to agree with what has been discussed here in the past.

This saved me some searching.

jtk

[george wilson]

Ball screws have recirculating ball bearing balls,can be made to nearly zero backlash,and therefore are the most ideal screws to date for CNC equipment where you will also be milling contours,and don't want "steps",flat spots,or noticeable surface defects on the surfaces you are milling. Excello made(makes?) some manual milling machines with ball screws. My co worker Jon had one,but unfortunately the table screws cranked in the opposite direction from the bridgeport types we were used to. So,he kept making mistakes,and traded it in for a Bridgeport type,even though the Canadian Excello was quite a high quality machine.

[Mark Baldwin III]

I didn't know that they got used on milling machines, although it makes sense to do so. Ball screws can carry tremendous loads quietly and with much less effort than a standard or Acme thread. They are fun to work on. These days I probably teardown or build one every week.

Ball screw talk aside, after seeing the picture above, I definitely get why you'd want the 90 degree thread for a wooden screw.

[Ray Gardiner]

I fitted ball screws to my mill, (part of an on-going cnc conversion project), and love em, silky smooth no backlash.

Anyway back to wooden threads, I finished the tap, 90 degree thread profile, 5 tpi and 2" diameter.... not quite as I wanted to, but a useful learning experience. I have a problem with change gears on the lathe, that means I can't do 3 tpi, until I can make some new change gears. The best I could do is 5 tpi, which is a little on the fine side for 2", probably be ok for 1" or 1 1/2"

I welded a 3/4 bar on the top and welded a 3/4" drive socket onto that so I can use a standard 3/4 drive wrench to turn it... it takes a fair bit of effort..

I had to machine the threads twice, the first time, I didn't think about what the root diameter of the thread was going to be, and ended up with a size that I had no easy way of drilling, so I re-machined the threads to suit a root diameter of 1 3/4" which I have a forstner bit for. That combined with the thread pitch gives me a clearance diameter of a little bit less than 2"...

There are three 8mm cutting slots spaced 120 degrees apart and the center is bored out for chip clearance. I still have to file some clearance on the cutting and leading edges.


Here is a picture taken before I milled the cutting slots. Just to get some idea of the scale, that's a 12" ruler and a 1 3/4" forstner bit

http://www.backsaw.net/pics/tap200.jpg

I haven't yet sharpened the cutting edges, but I wanted to see how it went is a bit of scrap pine
http://www.backsaw.net/pics/tap201.jpg

Close up of the cutter and thread

http://www.backsaw.net/pics/tap203.jpg

Thread quality is not too bad, I expect it will be a little better with some lubricant and sharpened cutting edges.

Now, I can make a thread box...

Regards
Ray

[george wilson]

Glad to see that you are able to machine your own taps,Ray. I don't know how large or powerful your lathe is,or IF it could muster the power to cut 3 threads per inch. If you could get it geared right,you might take the threads in smaller "bites" rather than machining a whole flank at a time. I have collected gears for my own lathe to do very coarse threads,which I could do in wood. I used a router held at a 45º angle in the toolpost for threading large (1" wide) threads at work. That 90º traditional wood thread angle does come in handy when doing that.

If I mention things like the function of ball screws in detail,it would be for the benefit of non machining readers here.

[jamie shard]

Very cool... are these too much work to consider making them for order??? :D

[george wilson]

Just the tool steel for a 2" tap would be pretty expensive,unless an inserted tool steel cutter could be employed,and that would be more work. Case hardening ordinary cold rolled steel would not be free of energy costs either.

[Ray Gardiner]

Very cool... are these too much work to consider making them for order??? :D

Hi Jamie, Short answer, yes, too much work. (and I'm supposed to be retired!) It's a pity that all the commercially available ones are the wrong profile.

But they aren't hard to make, see if you can find a local engineering firm and give them a drawing/sketch. Show them some pictures as well.

Hi George,
I have a 12x36 taiwanese lathe, (It looks like the Grizzly G4003). 3 phase motor retro-fitted with VFD and DRO. Don't know if it would have enough power to do 3 tpi though.
2" 01 drill rod from victor.net is about $4 per inch, I just used mild steel. You can get hydraulic tubing in sizes that would be suitable, that's probably the cheapest option.

Regards
Ray

[Chris Vandiver]

Here is a related link(hopefully it works);
http://www.workbenches.se/verkstadsbilder.php

Look at the botoom right hand photo.

[jamie shard]

Hi Jamie, Short answer, yes, too much work. (and I'm supposed to be retired!)

Ha, I thought so! It's beautiful work and beautiful work always seems so simple when you look at the final product.

[seth lowden]

Very coincidental- I was going to post a link to the youtube video on that site, Chris. I will instead post the link to the English version. The video is kind of cool. I have been interested in these benches for a while, since someone posted about his on Badger Pond.

http://www.workbenches.se/en/index.php

I do not have extensive experience, but on my 100+ year old commercially made bench, the screw has 60 degree threads, and I have not seen any antique taps that are not 60 degree. Looks like they were all filed with a triangle file to me, not a square file.

[george wilson]

Seth,look into the Diderot encyclopedia,written in the 18th.C.. You will see proper wood thread forms in drawings there. Some things declined in the 19th.C.,and I'm afraid wood thread forms were among them. Having a thread in wood that is thicker at its bottom,and not as tall proportionately to metal threads is perfectly logical,given the relative ease with which wood can chip out or shear off as compared to metal.

[Johnny Kleso]

Can you buy some high grade threaded acme rod and carbonise it ?

[Ray Gardiner]

Thanks for that link Seth,

I will repost the image here.. Interesting thread box.
http://www.backsaw.net/pics/tap301.jpg
Attribution.. The above image is from http://www.workbenches.se/en/index.php

The next one is a Peugeot Freres tap and thread box from Peter McBride 2" 3.5 tpi
http://www.backsaw.net/pics/tap302.jpg
http://www.backsaw.net/pics/tap303.jpg
I hope Peter doesn't mind me reposting them here.

The third one is a custom made 2" 4 tpi made for IanW.
http://www.backsaw.net/pics/tap304.jpg
Image thanks to IanW.

And just to round out the references here are a few companies offering vise kits with wooden screws.

http://www.bigwoodvise.com/index.cfm
http://www.lakeerietoolworks.com/shop/page/1?shop_param=

All the commercially available screws, taps, and thread boxes (that I have found) are 60 degree profile, not 90 degrees as they should be...
that's not to say they wont work well and last a long time, and, 60 degree threads are fine, but 90 degree is a bit better.

The other conclusion, is for a 2" diameter thread 2 to 4 tpi seems to be the range.

Hope that helps someone else looking for similar info.

Regards
Ray

[george wilson]

3 tpi is ideal for a 2" screw. These size threads are large enough that they have the width to not shear off easily,even at 60º. That is the size and thread that I made for years for all the workbenches at Williamsburg. They got worn out every several years,and I had to keep making them.